The present invention relates to a method of forming a semiconductor device and a method of forming a self-aligned refractory metal salicide layer over semiconductor diffusion layers and polysilicon layers.
In accordance with highly required scaling down of the semiconductors devices and increasing the density of integration, a design rule of 0.15-0.25 micrometers minimum size has now been on the application to form highly integrated semiconductor devices such as memory devices and logic devices. The increase in the density of the semiconductor device requires substantive reductions in length of the gate electrode and in width of diffusion layers as well as in thicknesses of the diffusion layers and the polysilicon layer as the gate electrode, thereby resulting in increase in electrical resistance of the semiconductor device. The increase in electrical resistance causes a substantive delay in transmission of signals on the circuits. For realizing the substantive reduction in the resistance, it is, therefore, required to form silicide layers, particularly refractory metal salicide layers, on the polysilicon gate electrode and on the diffusion layers of the single crystal silicon substrate. The refractory metal silicide layers are formed by silicidation of a refractory metal layer and a self-alignment technique.
A conventional method of forming a MOS field effect transistor with silicide layers will be described with reference to FIGS. 1A through 1E, which are fragmentary cross sectional elevation views illustrative of MOS field effect transistors in sequential steps involved in the first conventional fabrication method.
With reference to FIG. 1A, field oxide films 102 are selectively formed on a surface of a silicon substrate 101 by a local oxidation of silicon method, thereby to define an active region surrounded by the field oxide films 102. An ion-implantation of an impurity into the active region of the silicon substrate 101 is carried out to increase a withstand voltage thereof. A thermal oxidation of silicon is carried out to form a gate oxide film 103 on the active region of the silicon substrate 101. A chemical vapor deposition method is carried out to deposit a polysilicon film having a thickness of about 150 nanometers on an entire region of the silicon substrate, so that the polysilicon film extends over the field oxide films 102 and the gate oxide film 103. The polysilicon film is doped with an impurity such as phosphorus to reduce a resistivity of the polysilicon film. The polysilicon film is patterned by a photolithography technique in order to form a polysilicon gate electrode 104 on the gate oxide film 103. A chemical vapor deposition method is carried out to deposit a silicon oxide film on an entire region of the silicon substrate 101, so that the silicon oxide film extends over the polysilicon gate electrode 104, the active region of the silicon substrate 101 and the field oxide films 102. An anisotropic etching to the silicon oxide film is carried out to leave the silicon oxide film only on side walls of the polysilicon gate electrode 104, thereby to form side wall spacers 105 on the side walls of the polysilicon gate electrode 104. An ion-implantation of an impurity such as boron or arsenic into the active region of the silicon substrate 101 is carried out by using the polysilicon gate electrode 104, the side wall spacers 105 and the field oxide films 102 as masks, thereby to form impurity doped regions in upper regions of the silicon substrate 101. A heat treatment to the silicon substrate 101 is carried out at a temperature in the range of 800 to 1000.degree. C. to form source/drain diffusion layers 106 in the upper regions of the silicon substrate 101.
With reference to FIG. 1B, a sputtering method is carried out by sputtering a titanium target to deposit titanium on an entire region of the silicon substrate 101, thereby entirely forming a titanium film 107 having a thickness of 50 nanometers, so that the titanium film 107 extends over the polysilicon gate electrode 104, the side wall spacers 105, the source/drain diffusion layers 106 and the field oxide films 102.
With reference to FIG. 1C, a heat treatment to the silicon substrate 101 is carried out by use of a lamp anneal at a temperature in the range of 600 to 650.degree. C. in a nitrogen atmosphere under an atmospheric pressure for a time in the range of 30 to 60 seconds, thereby to cause both a titanium nitration reaction and a titanium silicide reaction, wherein nitrogen atoms are thermally diffused into the titanium layer 107 whereby the titanium film, except for bottom regions thereof on the source/drain diffusion layers 106 and the polysilicon gate electrode 104, are made into a titanium nitride film 112, whilst silicon atoms in the source/drain diffusion layers 106 and in the polysilicon gate electrode 104 are also thermally diffused into the bottom region of the titanium layer 107, whereby titanium atoms in the bottom region of the titanium layer 107 but only on the source/drain diffusion layers 106 and the polysilicon gate electrode 104 are silicided with the thermally diffused silicon atoms from the source/drain diffusion layers 106 and the polysilicon gate electrode 104. As a result, the bottom regions of the titanium layer 107 but only on the source/drain diffusion layers 106 and the polysilicon gate electrode 104 are made into titanium silicide layers 109. Namely, the titanium film 107 on the side wall spacers 105 and the field oxide films 102 are completely nitrated. The titanium silicide layers 109, therefore, extends on the bottom surface of the titanium nitride film 112 and over the source/drain diffusion layers 106 and the polysilicon gate electrode 104. The titanium silicide layers 109 has a C49-crystal structure hang a relatively high resistivity of about 60 .mu..OMEGA.cm.
With reference to FIG. 1D, the titanium nitride film 112 is completely etched by a wet etching which uses a chemical of a mixture of ammonium solution and a hydrogen peroxide solution, whereby the C49-structured titanium silicide layers 109 remain on the polysilicon gate electrode 104 and on the source/drain diffusion layers 106.
With reference to FIG. 1E, a heat treatment to the silicon substrate 101 is carried out at a temperature of about 850.degree. C. in a nitrogen atmosphere under an atmospheric pressure for 60 seconds to cause a phase transition from the C49 crystal structure to a C54 crystal structure which has a low resistivity of about 20 .mu..OMEGA.cm. As a result, the C49-structured titanium silicide layers 109 are made into C54-structured titanium silicide layers 111. Namely, the C54-structured titanium silicide layers 111 are formed on the polysilicon gate electrode 104 and on the source/drain diffusion layers 106.
The reason why the heat treatment for causing the silicidation reaction is carried out in the nitrogen atmosphere is as follows. During the heat treatment for causing the silicidation reaction of titanium with silicon, silicon may be diffused not only into the titanium film but also onto the silicon oxide films such as the field oxide films 102. The diffused silicon over the silicon oxide film is then reacted with titanium diffused from the titanium film, thereby to form a titanium silicide layer on an interface between the titanium film and the silicon oxide film. This titanium silicide layer will remain on the silicon oxide film such as the field oxide films, This means it no longer possible to obtain the required insulation. This phenomenon is so called to as "over-growth". In order to prevent the over-growth of the titanium silicide layer over the silicon oxide film, it is required to carry out the heat treatment for causing the silicidation reaction in the nitrogen atmosphere. The heat treatment in the nitrogen atmosphere causes a diffusion of nitrogen in the nitrogen atmosphere into the titanium film. Further, a nitration reaction temperature is lower than a silicidation Areaction temperature. This means that when the heat treatment is commenced and the temperature of the substrate is risen, then the nitration reaction is caused prior to the silicidation reaction. For those reasons, titanium in the titanium film over silicon having been diffused onto the silicon oxide film is fist reacted with nitrogen having been diffused from the nitrogen atmosphere into the titanium film, so that the titanium film over the silicon oxide film is nitrated with the diffused nitrogen from the nitrogen atmosphere, whereby almost no titanium atoms in the titanium nitride film over the silicon oxide film could be silicided with silicon on the interface between the silicon oxide film and the titanium nitride film. Therefore, no titanium silicide layer is formed on the silicon oxide film. This means that the heat treatment for causing the silicidation reaction in the nitrogen atmosphere allows the silicide layer to be self-aligned and selectively formed only on the silicon layer or the polysilicon layer.
The above technique for preventing the over-growth of the silicide layer on silicon oxide region is disclosed in the Japanese patent applications Nos. 7-303928 and 8-263906. This conventional technique will be described with reference to FIGS. 2A through 2E which are fragmentary cross sectional elevation views illustrative of MOS field effect transistors in sequential steps involved in the second conventional fabrication method.
With reference to FIG. 2A, field oxide films 102 are selectively formed on a surface of a silicon substrate 101 by a local oxidation of silicon method, thereby to define an active region surrounded by the field oxide films 102. An ion-implantation of an impurity into the active region of the silicon substrate 101 is carried out to increase a withstand voltage thereof. A thermal oxidation of silicon is carried out to form a gate oxide film 103 on the active region of the silicon substrate 101. A chemical vapor deposition method is carried out to deposit a polysilicon film having a thickness of about 150 nanometers on an entire region of the silicon substrate, so that the polysilicon film extends over the field oxide films 102 and the gate oxide film 103. The polysilicon film is doped with an impurity such as phosphorus to reduce a resistivity of the polysilicon film. The polysilicon film is patterned by a photolithography technique in order to form a polysilicon gate electrode 104 on the gate oxide film 103. A chemical vapor deposition method is carried out to deposit a silicon oxide film on an entire region of the silicon substrate 101, so that the silicon oxide film extends over the polysilicon gate electrode 104, the active region of the silicon substrate 101 and the field oxide films 102. An anisotropic etching to the silicon oxide film is carried out to leave the silicon oxide film only on side walls of the polysilicon gate electrode 104, thereby to form side wall spacers 105 on the side walls of the polysilicon gate electrode 104. An ion-implantation of an impurity such as boron or arsenic into the active region of the silicon substrate 101 is carried out by using the polysilicon gate electrode 104, the side wall spacers 105 and the field oxide films 102 as masks, thereby to form impurity doped regions in upper regions of the silicon substrate 101. A heat treatment to the silicon substrate 101 is carried out at a temperature in the range of 800 to 1000.degree. C. to form source/drain diffusion layers 106 in the upper regions of the silicon substrate 101.
With reference to FIG. 2B, a sputtering method is carried out by sputtering a titanium target to deposit titanium on an entire region of the silicon substrate 101, thereby entirely forming a titanium film 107 having a thickness of 20 nanometers, so that the titanium film 107 extends over the polysilicon gate electrode 104, the side wall spacers 105, the source/drain diffusion layers 106 and the field oxide films 102. Subsequently, a further sputtering method is carried out by sputtering a titanium nitride target to deposit titanium nitride on an entire region of the titanium film 107, thereby forming a titanium nitride film 113 which extends over the titanium film 107.
With reference to FIG. 2C, a heat treatment to the silicon substrate 101 is carried out by use of a lamp anneal at a temperature in the range of 600 to 650.degree. C. in an argon atmosphere under an atmospheric pressure for a time in the range of 30 to 60 seconds, thereby to cause both a titanium nitration reaction and a titanium silicide reaction. This heat treatment causes that silicon atoms in the source/drain diffusion layers 106 and the polysilicon gate electrode 104 are thermally diffused into lower regions of the titanium film 107, so that the lower regions of the titanium film 107 over the source/drain diffusion layers 106 and the polysilicon gate electrode 104 are silicided with the thermally diffused silicon atoms, whereby titanium silicide layers 109 are self-aligned and selectively formed on the source/drain diffusion layers 106 and the polysilicon gate electrode 104. The titanium silicide layers 109 has a C49 crystal structure having a relatively high resistivity of about 60 .mu..OMEGA.cm. On the other hands, upper regions of the titanium film 107 over the source/drain diffusion layers 106 and the polysilicon gate electrode 104 and further the upper and lower regions of the titanium film 107 over the side wall spacers 105 and the field oxide films 102 are nitrated with nitrogen atoms thermally diffused from the titanium nitride film 113 overlying the titanium film 107, thereby to form a nitrogen containing titanium film 114 which extends over the field oxide films 102, the C49-structured titanium silicide layers 109 and the side wall spacers 105. As a result, the bottom regions of the titanium layer 107 but only on the source/drain diffusion layers 106 and the polysilicon gate electrode 104 are made into the C49-structured titanium silicide layers 109. Namely, the titanium film 107 on the side wall spacers 105 and the field oxide films 102 are completely nitrated. The titanium silicide layers 109, therefore, extend on the bottom surface of the nitrogen containing titanium film 114 and over the source/drain diffusion layers 106 and the polysilicon gate electrode 104.
With reference to FIG. 2D, the nitrogen containing titanium film 114 is completely etched by a wet etching which uses a chemical of a mixture of ammonium solution and a hydrogen peroxide solution, whereby the C49-structured titanium silicide layers 109 remain on the polysilicon gate electrode 104 and on the source/drain diffusion layers 106.
With reference to FIG. 2E, a heat treatment to the silicon substrate 101 is carried out at a temperature of about 850.degree. C. in an argon atmosphere under an atmospheric pressure for 60 seconds to cause a phase transition from the C49 crystal structure to a C54 crystal structure which has a low resistivity of about 20 .mu..OMEGA.cm. As a result, the C49-structured titanium silicide layers 109 are made into C54-structured titanium silicide layers 111. Namely, the C54-structured titanium silicide layers 111 are formed on the polysilicon gate electrode 104 and on the source/drain diffusion layers 106.
In accordance with the above second conventional method, the titanium nitride film has been formed on the titanium film before the heat treatment is carried out in the argon atmosphere in order to prevent the over-growth of the titanium silicide layer over the field oxide films 102 or over the side wall spacers 105. Namely, the second conventional method is effective for allowing the titanium silicide layers 109 to be self-aligned only on the polysilicon gate electrode 104 and on the source/drain diffusion layers 106.
The second conventional method is, however, engaged with the following problems.
It order to realize the scaling down of the semiconductor devices, it is required to reduce a thickness of the titanium silicide layers. Notwithstanding, it is difficult for the conventional methods to form extremely thin titanium suicide layers on the source/drain diffusion layers and on the polysilicon gate electrode. In order to reduce the thickness of the titanium silicide layers, it is also required to reduce a thickness of the titanium film. During the heat treatment for causing the silicidation reaction, not only the silcidation reaction of titanium but also the nitration reaction of titanium are caused, wherein the silicidation reaction and the nitration reaction are exclusive to each other, and therein the temperature of beginning the titanium nitration reaction is lower than the temperature of beginning the titanium silicidation reaction. This means that the titanium nitration reaction is caused prior to the titanium silicidation reaction, and therefore, if the titanium film is extremely thin, the entire of the titanium film may be nitrated before the required silicidation reaction could no longer be caused even the temperature of the silicon substrate is risen up to the temperature of causing the titanium silicidation reaction. If particularly the silicon region contains arsenic as an impurity, then the silicidation reaction rate is reduced, whilst the nitration reaction is relatively promoted, whereby the extremely thin titanium film may entirely be nitrated followed by no silicidation reaction.
If, in accordance with the first conventional method, the heat treatment for causing the titanium silicidation reaction is carried out in the nitrogen atmosphere, this provides an influence to the phase transition of the titanium silicide film from the C49 crystal structure to the C54 crystal structure. FIG. 3 is a diagram illustrative of variation in phase transition temperature from the C49 crystal structure to the C54 crystal structure of the titanium silicide film over thickness of the titanium film. As the thickness of the titanium film is increased, the phase transition temperature is decreased. If the thickness of the titanium film is thicker than 30 nanometers, then the phase transition temperature is below 800.degree. C. It is preferable that the phase transition temperature is low. Particularly, as the thickness of the titanium film is reduced from 30 nanometers, the phase transition temperature is rapidly increased. The reason why the reduction in thickness of the titanium film results in increase of the phase transition temperature is as follows. The heat treatment for causing the silicidation reaction is carried out in the nitrogen atmosphere, whereby nitrogen atoms in the nitrogen atmosphere may thermally be diffused into the titanium film and a nitration reaction of titanium appears in the titanium film. If the titanium film is thin, then a nitrogen concentration of the thin nitrogen containing titanium film is high. If, however, the titanium film is thick, then a nitrogen concentration of the thick nitrogen containing titanium nitride film is low. The increase in concentration of the nitrogen containing titanium film results in an increase of the phase transition temperature, whereby the second heat treatment for causing the phase transition from the C49-structured titanium silicide layer into the C54-structured titanium silicide layer is required to be carried out at a high temperature. The heat treatment at a high temperature may provide influences to the source/drain diffusion layers, whereby characteristics of the semiconductor device are deteriorated. The heat treatment at a high temperature may also cause a cohesion of the titanium silicide layer, whereby a resistivity of the titanium silicide layer is increased.
In contrast to the above heat treatment in the nitrogen atmosphere, a heat treatment in an argon atmosphere for causing the titanium silicide layer is preferable in the light of suppressing an excess nitrogen diffusion and relatively activating the silicidation reaction. If the heat treatment is carried out in the argon atmosphere, then no nitrogen atoms are thermally diffused into the titanium nitride film 108 overlaying the titanium film 107, whilst nitrogen in the titanium nitride film 108 is diffused to the titanium film 107, whereby the nitrogen concentration of the titanium nitride film 108 is decreased. Further, a depth of nitrogen diffused into the titanium film 107 is shallower as compared to when the heat treatment is carried out in the nitrogen atmosphere. Namely, the heat treatment in the argon atmosphere suppresses an excess nitrogen diffusion into the titanium film 107, so that no nitration reaction is caused in the lower or bottom region of the titanium film 107, thereby allowing the silicidation reaction in the lower or bottom region of the titanium film 107 to form a titanium silicide layer. Consequently, the heat treatment in the argon gas atmosphere free of nitrogen allows a formation of the titanium suicide layer even if the titanium film 107 is extremely thin for scaling down of the semiconductor device.
As described above, the above second conventional method of forming the titanium silicide layer is made by forming laminations of the titanium film and the titanium nitride film for subsequent heat treatment in the argon atmosphere. This second conventional method has the following problems.
The titanium nitride film has been formed on the titanium film before the heat treatment is carried out to cause the silicidation reaction to form the C49-structured titanium silicide layer. The titanium nitride film may be sintered by this heat treatment. The sintering of the titanium nitride film increases a density of the titanium nitride film. The sintered titanium nitride film overlying the C49-structured titanium silicide layer is then required to be removed. Notwithstanding, it is difficult to remove the sintered titanium nitride film having a high density by a wet etching which uses a chemical which comprises a mixture solution of ammonium and hydrogen peroxide. Namely, sintering of the titanium nitride film or increase in film density of the titanium nitride film makes it difficult to remove the same by the wet etching. In place of the wet etching, over-etching or dry etching to the sintered titanium nitride film with the high film density may be considered for solving the above problem with difficulty in removal of the sintered titanium nitride film. However, either the wet etching or the dry etching are insufficient in etching selectivity of a titanium nitride etching rate to a titanium silicide etching rate. Namely, the titanium nitride etching rate is not sufficiently higher than the titanium silicide etching rate. For this reason, the thin titanium silicide layer is likely to be etched. This means it difficult to remove only the sintered titanium nitride film overlying the titanium silicide layer without etching the titanium silicide layer. The unintended etching to the titanium silicide layer results in variation in thickness of the remaining titanium silicide layer. This variation in thickness of the remaining titanium silicide layer results in increased variations in resistance of the titanium silicide layer. The over-etching to the thin titanium silicide layer makes it difficult to obtain a sufficiently reduced resistance of the remaining titanium silicide layer.
The sintering of the titanium nitride film by the heat treatment in the argon atmosphere for causing the silicidation reaction increases a film stress of the titanium nitride film. The titanium nitride film having the increased film stress has the following problems.
The silicon layer or silicon region, on which a titanium silicide layer is intended to be formed, is often surrounded by or bounded with the silicon oxide film or silicon oxide region. The silicidation reaction of a refractory metal such as titanium with silicon is initiated on an interface between the refractory metal film and the silicon layer or silicon region, whereby an initial refractory metal silicide layer is first formed on the interface between the refractory metal film and the silicon layer or silicon region. Thereafter, the silicidation reaction is continued to appear on an interface between the growing refractory metal silicide layer and the silicon layer or silicon region, whereby the refractory metal silicide layer is grown toward the silicon layer or silicon region. In the atomic level viewpoint, the continuous titanium silicidation reaction requires that silicon atoms are diffused onto the interface between the currently growing titanium silicide layer and the silicon layer or silicon region and further that the titanium atoms are immersed into the surface region of the silicon layer or silicon region, whereby the titanium silicide layer being on growth is immersed into the surface region of the silicon layer or silicon region. Namely, the titanium silicide layer is continuously grown with immersion thereof into the surface region of the silicon layer or silicon region. If the silicon layer or silicon region is surrounded by the silicon oxide region, then the immersion of the titanium silicide layer on growth into the surface region of the silicon layer or silicon region causes a plastic deformation toward the surface region of the silicon layer or silicon region. Since the titanium layer 107 overlies the titanium silicide layer being on immersion into the surface region of the silicon layer or silicon region, then the titanium layer also shows a plastic deformation together with the plastic deformation of the titanium silicide layer Since further the titanium nitride film overlies the titanium film, then the plastic deformation of the titanium film causes a further plastic deformation of the titanium nitride film. Those plastic deformations of not only the titanium silicide layer but also the titanium film and the titanium nitride film needs a larger force than a total film stress of the titanium silicide layer, the titanium film and the titanium nitride film. The provision of the titanium nitride film on the titanium film increases the required force for those plastic deformations of the titanium silicide layer, the titanium film and the titanium nitride film. This means that the provision of the titanium nitride film on the titanium film may provide a resistance or bar to the plastic deformations of the titanium silicide layer, the titanium film and the titanium nitride film. As described above, the continuous growth of the titanium silicide layer with immersion into the surface region of the silicon layer or silicon region needs the plastic deformations of not only the titanium silicide layer but also the titanium film and the titanium nitride film, for which reason the provision of the titanium nitride film on the titanium film may provide a resistance or bar to the continuous growth of the titanium silicide layer with immersion into the surface region of the silicon layer or silicon region. This resistance or bar to the continuous growth of the titanium silicide layer with immersion into the surface region of the silicon layer or silicon region reduces a rate of the titanium silicidation reaction, whereby the titanium nitrate reaction may exceed the titanium silicidation reaction. This may make it no longer possible to cause the required titanium silicidation reaction. Particularly, if the silicon layer or silicon region is defined by the silicon oxide regions to form a line with a reduced width, the required force for the plastic deformations of the titanium silicide layer, the titanium layer and the titanium nitride film is relatively high. This results in a further resistance or bar to the continuous growth of the titanium silicide layer with immersion into the surface region of the silicon layer or silicon region, resulting in a reduction in a rate of the titanium silicidation reaction, whereby the titanium nitrate reaction may exceed the titanium silicidation reaction. This may make it no longer possible to cause the required titanium silicidation reaction.
Consequently, the second conventional silicidation method is effective to suppress the excess nitration reaction and allows a reduction in thickness of the silicide layer but not applicable to silicide the line-shaped silicon layer or silicon region.
In the above circumstances, it has been required to develop a novel method of forming a self-aligned refractory metal silicide layer on a silicon region surrounded by a silicon oxide region free from the above problems with the first and second conventional methods.